1. Earthquakes
Earth Science Unit 12

2. Why & How
So…why do earthquakes happen?
Tectonic plates are always moving
The boundary between 2 tectonic plates is
called a fault
Faults are areas where earthquakes
happen…how?

3. Why & How
The rocks on each side of the fault are moving
slowly.
As they move, sometimes they get stuck. If they
stay stuck for a while, a lot of pressure builds up.
When the rocks finally get un-stuck they break
(fracture) and move again, and the rocks
“rebound” past the place where they were stuck
and keep moving slowly past each other.
This is the Elastic Rebound Theory

4. Mechanics of an Earthquake
Where the plates actually get stuck is where the
earthquake begins…called the focus.
The point on Earth’s surface directly above the
focus is the epicenter.
Which plate
boundary would
have deeper focus
earthquakes?

5. Seismic Gaps
Sometimes the plates
get stuck for a long time
Why could this be bad?
We identify these places
because they haven’t
released any seismic
waves in a long time.
This is a “gap” in the
record of seismic
activity

6. What makes an Earthquake
Dangerous?
So, plates get stuck and then move quickly…what
does this really do?
This creates vibrations (waves) that travel through
the ground.
What does a wave look like?

7. Seismic Waves
There are 4 types of seismic waves released in an
earthquake.
The first two move deep within the Earth, and are
called Body Waves
The second two move through the surface of the
Earth and are called Surface Waves

8. Seismic Waves
Use Arrows Amount of
Time in Describe
Wave to Describe Damage
Earthquake Movement
Movement Caused
P Wave
S Wave
Love
Wave
Rayleigh
Wave

9. Where
What kinds of plate boundaries usually
create earthquakes?

11. Quick Quiz #1
The place underground where an earthquake
actually begins.
The seismic wave that arrives first in an
earthquake
Love and Rayleigh waves are the two types
of ____________ waves.
Why is a seismic gap dangerous?

12. Where
What kinds of plate boundaries can create
earthquakes?
ANY kind of plate boundary!
Boundaries are places where tectonic
plates rub against each other.
How are their earthquakes different?

13. Transform Boundary Earthquakes
Form where the plates move past one another
Usually begin near the surface…why?
Create lots of vibrations near the surface, which
cause a lot of damage
Largest in the world is along the coast of
California

14. Convergent Boundary Earthquakes
Where one plate subducts beneath another plate
Usually begin deep underground…why?
Their vibrations are felt over a large area
If they are totally underwater, they form:
 Underwater volcanoes, which can make island arcs
 Tsunamis
 Example: Aleutian Islands
If they are on land, they form:
 Terrestrial volcanoes
 Example: West coast of South America

15. Underwater Convergent Boundary
Aleutian Islands, Alaska

16. Terrestrial Convergent Boundary

17. Terrestrial Convergent Boundary
Andes Mountains, South America

18. The Pacific Ring of Fire
One of 3 major
earthquake
zones in the
world
Surrounds the
Pacific Ocean
Contains
Transform &
Convergent
boundaries.

19. The Eurasian-Melanesian
Mountain Belt
 One of the 3 major earthquake zones in the world
 Mountains that extend from Europe into Asia
 Contains mostly Convergent Boundaries

20. Divergent Boundary
Earthquakes
Form where the plates
spread apart
Usually begin near the
surface
Usually too small to be felt
by people
Most found on the Ocean
Floor

21. The Mid-Atlantic Ridge
One of 3 major earthquake zones in the world
Splits the Atlantic Ocean
Mostly underwater

22. Magnitudes of Earthquakes
 The Richter Scale: Measures Energy Released
 Measures from 1-12, with 1 being the smallest and
12 being the highest.
 Major Earthquake: over 7
 Moderate Earthquake: 6-7
 Minor Earthquake: 2.5-6
 Microquake: less than 2.5 (cannot be felt by
people)
 The Mercalli Scale
 Measures in Roman numerals from 1-12 (I – XII)

23. The Richter Scale

24. The Richter Scale

25. Mercalli Scale

26. Magnitudes of Earthquakes
The largest earthquake ever recorded was
estimated at 9.6 on the Richter scale, and
caused massive damage. It is the only
earthquake ever recorded over 9.
Most major earthquakes are between 7 and
8, and most scientists believe only 1 or 2
earthquakes over 8 will happen in 100 years.
Most scientists do not believe that an
earthquake larger than 10 on the Richter Scale
could ever happen.
On the Richter Scale, the highest number (12)
is described as “total destruction of the
Earth, where the Earth would be split in two.

27. Finding Epicenter & Magnitude
 http://www.sciencecourseware.org/virtualearthquake/VQuake
Execute.html

28. Quick Quiz #2
The deepest earthquakes are found at
________________ boundaries.
The west coast of North America is part of an
earthquake zone known as the
___________________.
To find the epicenter of an earthquake, you need
to know the arrival time of the ______ wave from
3 recording stations.
A ________________ earthquake has a Richter
magnitude between 6 & 7.